In general, it is desired that pneumatic tires suitable for snowy roads, including roads with compacted snow, have excellent braking performance (on-snow braking performance), traction performance (on-snow traction performance), and turning performance (on-snow turning performance) on snowy roads.
Normally, the braking performance, traction performance, turning performance, and the like of a tire are affected by the friction characteristics of the tire. Therefore, in order to improve the on-snow braking performance, on-snow traction performance, and on-snow turning performance of a tire, it is necessary to improve the friction characteristics of the tire on snowy roads.
One known way of improving the friction characteristics of a tire on snowy roads is to enhance the shear resistance of the snow column formed in a groove provided in the tread (i.e. increase the snow column shear force) by improving the force with which the tire grabs and packs trodden snow when the tire is rotated with load applied thereon. Other known ways of improving the friction characteristics of a tire on snowy roads include guaranteeing the tire ground contact area, as well as enhancing the scratch effect on the road surface due to edges of blocks and edges of sipes formed in the tread.
In a conventional pneumatic tire, however, in which a plurality of main grooves extending along the tire circumferential direction and a plurality of lateral grooves extending along the tire width direction are formed on the tread surface to define rectangular blocks, with sipes formed in the blocks, it has not been possible to achieve an improvement in snow column shear force, a guarantee of tire ground contact area, and an improvement in the scratch effect on the road surface simultaneously.
In other words, in the above conventional pneumatic tire, upon increasing the groove depth of the lateral grooves in order to enhance the snow column shear force, the rigidity of the block in the tire circumferential direction decreases, and the angle of collapse of the block upon ground contact ends up increasing, thus making it impossible to guarantee the tire ground contact area. Furthermore, if the number of grooves or sipes formed in the tread surface is increased in order to enhance the scratch effect on the road surface due to edges of blocks and edges of sipes, the rigidity of the block decreases, and the angle of collapse of the block upon ground contact ends up increasing, thus making it impossible to guarantee the tire ground contact area.
On the other hand, in order to guarantee the tire ground contact area by suppressing the decrease in rigidity of the block, it is necessary to form rib-shaped land portions instead of blocks in the tread surface or to suppress the sipe density to be within a predetermined range (for example, see JP2001-71713A (PTL 1)), thus making it impossible to enhance the snow column shear force and the scratch effect on the road surface.